1. Field of the Invention
This invention relates to a converter circuit powered by a constant current source. It is particularly concerned with a converter circuit powered by a coaxial cable where the length of the cable may vary considerably. It is specifically concerned with powering the converter circuit with a power source having a constant current but a variable or unstable voltage.
2. Prior Art
Telephone systems many times require the location of a power supply at some remote site. In many instances commercial AC or site located battery power is unavailable at these remote sites. Hence, power must be transmitted to the remote site from a central office location. In some converter applications the power to energize the power supply or converter at the remote site may be supplied by a cable which is coupled to a power source at a distant central office. The length of this cable varies according to the location of the central office with respect to the converter at the remote site. It is not unusual in some instances for the length of a cable from a central office to a remote site to vary from as little as a couple of miles to distances in excess of thirty miles.
Due to the electrical characteristics of coaxial cables, the power supplied through the cable to the converter has the characteristics of a constant current source. Inasmuch as the length of the cable may vary considerably, the voltage at the converter input may vary considerably since the voltage magnitude at the remote end of the cable is a function of the varying cable length. It is not unusual for the voltage applied to the input of a converter at one remote site to vary by as much as a factor of two from the voltage input at another remote site. Due to the relatively wide variations in input voltage it may not be feasible in many instances to use a conventional DC to DC converter source for an application such as described. By way of contrast, most DC to DC converter applications require a converter having a regulated output, whereas the input voltage is assumed to be relatively constant.
Since conventional converters are designed to operate with a relatively constant input voltage, variation of the input voltage from this selected value due to widely varying cable lengths may produce adverse effects on the efficiency and regulation characteristics of a conventional converter.
A typical conventional converter utilized in a remote location and powered by a constant voltage source is shown connected to a coaxial cable in FIG. 1. A cable C of variable length couples a voltage source V.sub.s located at a central office to a DC to DC converter located at some remote site. Since the length of the cable may vary, it is shown to have a variable resistance R.sub.c whose magnitude is a function of the cable length. The circuit arrangement includes a dissipating power resistance R.sub.p utilized to dissipate power due to the higher voltage applied to the converter at the shorter cable lengths.
Due to the operating characteristics of conventional converters as described above, the regulation and efficiency of this converter powering arrangement is poor since, due to variations in the length of the cable in different applications, it is not unusual to have the input voltage vary by a factor of two in different site locations. Because of the instability of the input voltage due to variation in cable length, the converter must be designed to regulate over a wide range of input voltages on the order of a factor of two. Furthermore, at short cable lengths, the increased input voltage places a high voltage stress on the circuit components of the converter.
One solution of the prior art to this variable length cable problem has been to utilize a series dissipation resistance such as the resistance R.sub.p shown in FIG. 1 and, in some instances, shunt regulators are used at the input of the converter. While this arrangement improves the regulation of the converter, it operates to dissipate excess power at shorter cable lengths. This dissipation of power affects efficiency adversely. The shunt regulator must be designed to operate over a wide range of voltages and be capable of dissipating large amounts of power.